Laser Rayleigh scattering

Rayleigh scattering is a powerful diagnostic tool for the study of gases an d is particularly useful for aiding in the understanding of complex flow fi elds and combustion phenomena. Although the mechanism associated with the s cattering, induced electric dipole radiation, is conceptually straightforwa rd, the features of the scattering are complex because of the anisotropy of molecules, collective scattering from many molecules and inelastic scatter ing associated with rotational and vibrational transitions. These effects c ause the scattered signal to be depolarized and to have spectral features t hat reflect the pressure, temperature and internal energy states of the gas . The very small scattering cross section makes molecular Rayleigh scatteri ng particularly susceptible to background interference. Scattering from ver y small particles also falls into the Rayleigh range and may dominate the s cattering from molecules if the particle density is high. This particle sca ttering can be used to enhance flow visualization and velocity measurements , or it may be removed by spectral filtering. New approaches to spectral fi ltering are now being applied to both Rayleigh molecular scattering and Ray leigh particle scattering to extract quantitative information about complex gas flow fields. This paper outlines the classical properties of Rayleigh scattering and reviews some of the new advances in flow field imaging that have been achieved using the new filter approaches.